1. Field of the Invention
The present invention concerns a process and an apparatus for shaping by multiaxial compacting carbonaceous blocks and more particularly but not exclusively anodes intended for the production of aluminium by electrolysis using the Hall-Heroult process.
2. Description of Related Art
At the present time, carbonaceous blocks such as the anodes which are used for the production of aluminium are produced by ramming, pressing, vibro-tamping or vibro-pressing of a carbonaceous paste formed by a crushed coke (and/or anthracite) aggregate and pitch or tar.
Those four procedures are based on the same principle: the carbonaceous paste is introduced at a temperature of the order of 120.degree. to 140.degree. C. into a non-deformable mould (referred to as `oedometric`) and a stress or a succession of substantially vertical stresses is or are applied thereto.
The application of the stress, referred to as `loading`, may be a single operation, which is the case with the pressing process in general, or a multiple operation, with two or three successive pressing operations in certain cases, and up to several hundreds of times in the case of vibro-tamping.
The levels of force used in different situations are substantially different: for example 28 MPa for a press with a single cycle, 2.times.18 MPa for a press with a double cycle, and 0.5 to 2.5 MPa in the case of a vibro-tamper. However, in every case, the level of compacting is limited either by the tar content, upon reaching a condition of saturation with tar of the intergranular space in the coke aggregate, or, in the case of a tar content which is less than the condition of saturation, the level of compacting is limited by virtue of the fact that the grains of coke come into contact with each other and cannot be brought any closer together. Therefore, in regard to uniaxial compacting, that is to say with a force applied in a single direction, there is a physical limit which prevents the apparent density of the compacted material from going beyond a certain value.